This project investigates the biological and physicochemical properties of a unique class of powerful immunological adjuvants. These compounds, which are surface active agents, appear to act via a previously unsuspected mechanism which involves an ability to form multivalent binding sites on hydrophobic surfaces. We tested the ability of 10 large hydrophobic surfactants to enhance antibody formation and inflammation. The surfactants were all block copolymers of 10 or 20% hydrophilic polyoxyethylene (POE) and the rest hydrophobic polyoxypropylene (POP). They differed in molecular weight and mode of linkage of POP to POE. Each of the 10 agents produced a distinct pattern of immune response and inflammation. For example, surfactants with 10% POE and the structure POE-POP-POE were effective adjuvants for antibody formation and produced moderate inflammation at the site of injection. Those with similar composition but the reverse structure of POP-POE-POP were poor adjuvants, but produced severe inflammation. The activity of all the agents was dependent upon presentation of the material on a surface. These data demonstrate that hydrophobic, nonionic surfactants have a spectrum of biological activities which are not dependent on the chemical nature of their constituent parts. We suggest that the activity of these agents derives from their ability to pack together as 'macro active' sites on the surface of oil drops. These active sites influence the concentration and conformation of both antigen molecules and host proteins in diverse ways to produce their biological effects. These studies are developing more powerful and precise ways to stimulate particular types of immune responses to diverse antigens.